Pulse generator



J. C- MARTIN PULSE GENERATOR Dec. 21, 1965 3 Sheets-Sheet 1 Filed Jan. 7, 1963 h P 6 M mw w? x3 55 4 QJL/ mm. 7% Z #2 mg 1 3 JP J. C. MARTIN PULSE GENERATOR Dec. 21, 1965 Filed Jan. 7, 1965 3 Sheets-Sheet 2 J. C. MARTIN PULSE GENERATOR Dec. 21, 1965 Filed Jan. 7, 1963 3 Sheets-Sheet 3 United States Patent 3,225,223 PULSE GENERATGR John Christopher Martin, Boundary Hall, Tadley, Hampshire, England United Kingdom Atomic Energy Authority, 11 Charles II St., London SW. 1, England) Filed Jan. 7, 1963, Ser. No. 249,873

Claims priority, application Great Britain, Jan. 12, 1962,

8 Claims. (Cl. 367-408) This invention relates to high-voltage pulse generators employing pairs of transmission lines which are connected to be charged in parallel but discharged in series through a load. Such generators employing cables are known as Blumlein circuits. A description of the basic Blumlein circuit is given, for example, in J. Inst. Electrical Engineers, vol. 93, pt. IIIA (1946) at page 1098.

It is possible, in theory to multiply the output voltage of such pulse generators by connecting in series a number of unit Blumlein circuits corresponding to the requisite multiple of their individual output voltages. In practice however the gain achieved is reduced by a number of effects. These effects include corona losses, resistance effects in metallic conductors, pulse overlap between adjacent transmission lines, losses due to stray capacitance and losses due to the initial charging process itself. The reduced gain is due not only to the above effects but also to what may be termed back impedance. This back impedance is an impedance which the multiple circuit would present to the output pulse in parallel with the load, and is found to affect the attained output voltage V under open-circuit conditions according to the expression out X It will be seen that the larger the value of R relative to Z0, the more closely V approaches the theoretical open circuit value of 2nV.

When a load R is connected to the circuit the above expression becomes R R RBRL+2 (RB TLZU where R =impedance of the load.

It is found in practice that the value of R is very dependent on the configuration and layout of the transmission lines comprising the multiple Blumlein circuit. In particular it has been found that, using the coaxial lines and layouts heretofore employed, satisfactory multiple circuits have not been attainable.

It is one object of the present invention to provide a form of unit pulse generator circuit better adapted to be connected in series with other such units.

In its broadest aspect the invention comprises a unit pulse generator circuit comprising a pair of superimposed parallel strip transmissioin lines, the pair being composed of two outer conductors and one inner conductor, means for charging the transmission lines so that their electric fields are opposed and the said outer conductors have substantially the same polarity, a switch for short circuiting one end of one transmission line at one end of the said pair and output connections from the said outer conductors at the other end of the said pair.

Also according to the present invention a unit pulse generator circuit comprises at least three metal foils arranged in spaced parallel relationship to form at least 3,225,223 Patented Dec. 21, 1965 one pair of parallel strip transmission lines, each pair of lines sharing a common foil, output connections from adjacent ends of two of the foils other than the common foil, and switch means for short-circuiting the other end of one of said pairs of lines.

Said two foils are preferably arranged on opposite sides of said common foil, and the unit may comprise two such pairs of lines arranged symmetrically on opposite sides of one of the non-common foils, corresponding foils being interconnected so that the two corresponding lines of each pair are electrically in parallel with one another.

A multiple pulse generator circuit according to the present invention comprises a plurality of units as aforesaid arranged in spaced parallel relationship and having the outputs connected in series with one another.

To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the drawings accompanying the present specification wherein FIG. 1 is a diagrammatic representation of a unit pulse generator circuit embodying the present invention.

FIG. 2 is a diagrammatic representation of a multiple (three-unit) pulse generator.

FIG. 3 is a modification of the circuit of FIGURE 2.

FIG. 4 is a diagrammatic illustration of a two unit pulse generator circuit.

Referring to FIG. 1 there is shown a pair of parallelstrip transmission lines 1 and 2 comprising two foils 4 and 5 and a common foil 6 folded back in itself. It will be understood that the foils are shown diagrammatically by lines representing one of their longitudinal edges, the insulation between the foils being omitted for simplicity. Lines 1 and 2 have the same characteristic impedance Z0 and the same transit time t. (The transit time is the time taken by a pulse front to travel from one end to the other.) The output is taken from adjacent ends of the foils 4- and 5 to a further parallel-strip transmission line 3 (formed by continuations of foils 4 and 5) having an impedance 2-Z0, to the other end of which is connected a matched load R -=2Z0 which is shown as a resistor but may be an oscillator tube, an X-ray tube, or any other device utilizing high-voltage pulses. The other (non-output) end of foil 4- is connected via switch means S to adjacent end of the common foil 6. Provision is made for charging the lines 1 and 2 from a capacitor 4 via capacitor discharge switch and resistors R.

In operation the lines 1 and 2 are both charged to a voltage V with the switch S open. On S being closed to short-circuit the non-output end of line 1, a pulse of voltage V travels from left to right along line 1 until it reaches the junction with lines 2 and 3. Applying Kirchoffs Law as in the abovementioned reference it can be shown that the following relationships hold:

/3= /2 where B is the fraction of the pulse reflected back down line 1 :1 where 'y is the fraction of the pulse voltage impressed on line 3, and 6= /2 where 5 is the fraction of the pulse which passes into line 2.

Therefore after a transit time interval t line 2, and thereafter the matched load R has impressed upon it a voltage V.

The reflected fraction of the pulse travels as a pulse V/2 back down line 1 and is reflected after a time t as a pulse +V/2 at the short circuit at S, line 1. This pulse travels to the right and discharges line 1. The fraction of the pulse passing into line 2 travels down the line as a pulse -V/2 until after a time t it is reflected at the open circuit as a pulse V/2. This pulse travels to the right and discharges line 2. The +V/2 pulse and the V/2 pulse meet at the junction after a time 2t from their initiation at the junction and cancel each other. This then brings the volts on the line 3 down to zero. The total time that V is impressed on the load is thus 21.

The use of parallel-strip lines enables the value of Z to be made very low as compared with coaxial lines, e.g. about 0.1 ohm, and hence, for a given value of R tends to increase V The value of R in a multi unit generator is found to approximate to the characteristic impedance of a strip line formed between foils 4 and neglecting the intervening foils; hence R increases with the separation of foils 4 and 5, i.e. effectively with the spacing between the two halves of foil 6.

In one embodiment of the circuit of FIG. 1 lines 1 and 2 were formed of copper foil separated by polyethylene. One foil (foil 5) 5 metres long and 90 cm. wide was laid flat and on it was laid a similar size sheet of polyethylene. Another foil 4 metres long (foil 6) was folded back on itself to form two portions separated by a sheet of /2" thick polymethylmethacrylate. This folded foil was laid on the polyethylene layer and covered by another layer of polyethylene of similar size, on which was laid a further copper foil (foil 4) the same size as foil 5. In this way the three transmission lines 1, 2 and 3 are produced having impedances of 0.1 ohm, 0.1 ohm and 0.2 ohm respectively. R was made 0.2 ohm.

The switch S was of the kind described in copending application No. 799/62 and comprised a coplanar array of 50 blind holes about deep stabbed in A polyethylene. Lines 1 and 2 were charged in parallel to 100 kv. by a pulse of 1p sec. duration, at which voltage the stabbed polyethylene broke down thus shorting the switch S. The time taken for the switch to operate was only 3X secs., the rate of rise of current being 5 l0 a./sec. and the inductance 3 lO h. The pulse propagated down line 3 had a voltage of 100 kv. a steep leading edge of e-folding time (i.e. the time for the volts to rise to 1e of the final volts) of the order of 1O sec. and a duration of 3 10- sec. (The e-folding time T of the leading edge is given by T=L/ Z0 where L is the inductance of the switch.)

FIG. 2 shows a multiple (3-stage) pulse generator circuit using three uni-ts similar to those shown in FIG. 1 connected in series. The units are modified to the extent that the doubled-over foil 6 of FIG. 1 has become the single foils 6A, 6B, and 6C in FIG. 2. This is equivalent to dispensing with the methylmet'hacrylate insulating sheet between the two portions of foil 6 and is permissible despite the two ends of foil 6 being apparently short-circuited because the short circiut is isolated from the load for a time 2t. It will be seen that the unit pulse generator circuits of the invention are connected in series by connecting together the adjacent outer foils of adjacent pairs of units (i.e. foils 5A and 4B and foils 5B and 4C) at their output ends. In practice these connections are made by using doublelength foils doubled over on themselves, cf. foil 6 in FIG. 1. The output connections to line 3 are taken from the output ends of the two outermost foils 4A and 5C. Common, low-impedance, charging connections of one polarity, for pulse charging, are made to the other (non-output) ends of foils 4A, 4B and 4C and to the output end of foil 5C via an additional foil 7; similar charging connections of the other polarity are made to foils 6A, 6B and 6C. Foil 7 which is the charging connection for line 2C is necessary because a charging connection to the non-output end of foil 5C would connect that end to foil 4C so that lines 1C and 20 would discharge in parallel instead of in series; for similar reasons insulation must be maintained between foils 5A and 4B, and between foils 5B and 4C. If D.C. rather than pulse charging were employed, foil 7 would not be 4 required because the non-output ends of foils 5C and 4C could be isolated from one another by charging resistors. Although in FIG. 2 a single common shorting switch S is shown in practice switches similar to these hereinbefore referred to, and described in said copending application, comprising a trigger foil, between two such sheets of stabbed polyethylene, can be inserted between foils 4A and 6A, and 4B and 6B and 4C and 6C in the positions designated by X, and triggered simultaneously by an external pulse.

The value of R in the circuit of FIG. 2 is approximately the characteristic impedance of the parallel-strip line found between foils 4A and 7 neglecting the intervening layers of foil and dielectric. It has been found that other losses occur due to antiphase pulses generated in a multi-unit generator. These losses can be significantly reduced by, in effect, folding each unit along its length. In practice the units are not formed by folding large sheets or foils, instead separate foils are used and low-impedance strip connections are made at the appropriate ends, so providing a geometrically symmetrical structure comprising two pairs of lines having the corresponding lines of the two pairs electrically connected in parallel.

FIG. 3 shows one embodiment of this kind (two stages only being shown for simplicity) in which foils 6A and 5A of FIG. 2 are repeated, as foils 6A and SA on the opposite side of foil 4A, to provide, in effect, two further strip lines 1A and 2A. These lines are connected in parallel with corresponding lines 1A and 2A respectively by strip connections between the foils at appropriate ends. A side strap 9A (shown diagrammatically) joins foils 5A and 5A at the output ends and foil 6A is continuous with foil 6A at the non-output end. Two switches X are connected between foil 4A and foils 6A and 6A at the non-output end, although when the foils are interconnected at the switch end, only one such switch is strictly essential. The B unit circuit is con nected similarly. In this embodiment the line 3 is dispensed with, the output from foils 4A and 513 being connected directly to the load R In FIGURE 4 one of the two unit generators com prises a pair of strip transmission lines formed by copper sheets 8, 9 and 8, 48 (8 being the common sheet) and the other unit generator comprises a pair formed by sheets 4%, 33 and 40, 47 (40 being the common sheet) respectively. Polyethylene insulation is provided between the sheets of each pair of lines. Sheets 9 and 33 are separated by a block of polymethylmethacrylate 42 and are connected together at one end by a copper sheet 43, sheets 9, 33 and 43 conveniently being formed of a single sheet of copper. Sheet 43 forms the series connection between the two unit generators, the output being taken from between the ends of sheets 47 and 48 as shown.

Sheets 8 and 40 are interconnected by a copper sheet 21, and sheets 9 and 47 by a copper sheet 3t). Sandwiched between sheets 21 and 30, to form a switch of the kind described in copending application Serial No. 249,853 filed January 7, 1963, are, starting from sheet 30, a polythene sheet 29, a copper sheet 45 on which rests a copper disc 25, a polythene sheet 23 having a plurality of blind transverse channels 24 in the region of the disc, and a second copper disc 22. Leads 26 and 27 are taken from disc 25 to discs 17 and 28 (not visible) respectively, located between sheets 8 and 9, and 40 and 47 respectively. Discs 17 and 28 are components of two further switches of the kind described in the aforementioned application. Pulse-charging connections 44 and 46 are taken from sheet 21 and sheet 47 respectively to a capacitor 56 which is charged by a Cockcroft-Walton generator (not shown) and discharged into the pulse generator by lowering the sphere 53 to form the center electrode of a spark-gap whose other two electrodes are shown as 51 and 52. A charging connection 50 having a high inductance in the time-scale of the generator output pulse but a low inductance in the time-scale of the charging pulse is made between sheets 9 and 48.

In operation, breakdown of the gap 51, 52, 53 charges the two pairs of transmission lines and eventually causes breakdown of the switch incorporating disc 25. Pulses transmitted from the latter disc to discs 17 and 28 act to short circuit simultaneously the non-output ends of the transmission lines formed by sheets 8, and 9, and 40 and 47 respectively, and to generate an output pulse between the output ends of sheets 47 and 48 in the manner already described.

An example of a generator of the invention had the following characteristics:

The transmission lines were formed of copper sheet separated by polyethylene sheet of thickness A The +ve copper sheet was 30 cms. wide and the ve sheet was 28 cms. wide. Each unit had its own switch S.

Number of unit generators=10.

Output impedance per unit=4 ohms. Total impedance of units=40 ohms. Back impedance of generator=105 ohms. Internal impedance of generator=30 ohms. Impedance of each switch=2 ohms. e-folding time: 1.5 X sec.

Pulse duration=3 X10 sec.

Observed gain: 12.5.

Charging voltage=200 kv.

Output voltage=2.5 mv. open circuit.

The gain of a 10 unit Blumlein circuit pulse generator using cables could not be more than 4 due to back impedance alone.

I claim:

1. A unit pulse generator circuit comprising at least three metal foils arranged in spaced parallel relationship to form at least one pair of superimposed parallel strip transmission lines, each pair of lines sharing a common coil, connections for charging said lines so that the two foils other than the common foil have the same polarity relative to the common foil, output connections from adjacent ends of said two foils other than the common foil and switch means for short-circuiting the end of one of said pairs of lines remote from said output connections.

2. A circuit as claimed in claim 1 wherein said two foils are arranged to face opposite surfaces of said common foil.

3. A circuit as claimed in claim 2 wherein two such pairs of lines are arranged symmetrically on opposite sides of one of said non-common foils, corresponding foils being interconnected so that the two corresponding lines of each pair are electrically in parallel with one another.

4. A multiple pulse generator circuit comprising a plurality of unit circuits as claimed in claim 2 arranged in superimposed spaced parallel relationship and having the outputs adjacent and connected in series with one another.

5. A multiple pulse generator circuit comprising a plurality of unit circuits as claimed in claim 3 arranged in superimposed spaced parallel relationship and having their outputs adjacent and connected in series with one another.

6. A multiple-unit pulse-generator circuit comprising a plurality of unit circuits, each unit circuit comprising at least three metal foils arranged in spaced parallel relationship to form at least one pair of superimposed parallel-strip transmission lines, each pair of lines sharing a common foil, output connections from adjacent ends of said two foils other than the common foil and switch means for short-circuiting the end of one of said pairs of lines remote from the output connections, said unit circuits being arranged in superimposed spaced parallel relationship with one another with their output connections adjacent and connected in series, connections for charging said lines so that the two foils other than the common foils of said unit circuits have the same polarity relative to the common foils of said unit circuits, and means for operating theswitch means of all said unit circuits substantially simultaneously.

7. A multiple-unit pulse-generator circuit as claimed in claim 6 wherein said two non-common foils of each unit circuit are arranged to face opposite surfaces of said common foil of said unit circuit.

8. A multiple-unit pulse-generator circuit as claimed in claim 6 wherein each unit circuit comprises two pairs of superimposed parallel-strip transmission lines, the non-common foils of each pair of lines being arranged to face opposite surfaces of the common foil of that pair, said pairs of lines being arranged symmetrically on opposite sides of a said non-common foil which is common to both pairs of lines, corresponding foils being interconnected so that the two corresponding lines of each pair are electrically in parallel with one another.

References Cited by the Examiner UNITED STATES PATENTS 2,411,140 11/1946 Lindenblad 307-106 X FOREIGN PATENTS 880,525 10/1961 Great Britain. MILTON O. HIRSHF IELD, Primary Examiner, 

1. A UNIT PULSE GENERAOR CIRCUIT COMPRISING AT LEAST THREE METAL FOILS ARRANGED IN SPACED PARALLEL RELATIONSHIP TO FORM AT LEAST ONE PAIR OF SUPERIMPOSED PARALLEL STRIP TRANSMISSION LINES, EACH PAIR OF LINES SHARING A COMMON COIL, CONNECTIONS FOR CHARGING SAID LINES SO THAT THE TWO FOILS OTHET THAN THE COMMON FOIL HAVE THE SAME POLARITY RELATIVE TO THE COMMON FOIL, OUTPUT CONNECTIONS FROM ADJACENT ENDS OF SAID TWO FOILS OTHER THAN THE COMMON FOIL AND SWITCH MEANS FOR SHORT-CIRCUITING THE END OF ONE OF SAID PAIRS OF LINES REMOTE FROM SAID OUTPUT CONNECTIONS. 